/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
 //判断链表是否有环
 //思路：快慢指针
class Solution {
public:
    bool hasCycle(ListNode *head) {
        if(!head)
            return false;
        struct ListNode* slow = head;
        struct ListNode* fast = head;//快指针在前即可
        while(fast && fast->next){
            fast = fast->next->next;
            slow = slow->next;
            if(fast == slow)
                return true;          
        }
      return false;
    }
};